Wind turbines create power proportional to the swept area of their blades. Increasing the length of a wind turbine's blades increases the swept area, which produces more power. A wind turbine's generator, gears, bearings, and support structure must be designed around the expected wind load and power production. At low wind speeds very long blades are desirable to get as much power as possible out of the available wind. At high wind speeds a wind turbine must control the power production and the mechanical loads developed. Eventually, if the wind speeds become high enough, the turbine must shut down to avoid damaging components, so short blades are desirable to keep the turbine producing power in high winds.
The choice of a rotor diameter for a wind turbine is a design trade-off between energy production in low winds and load limitation in high winds. Wind turbine manufacturers often sell a variety of rotor sizes for a given wind turbine model. The rotor sizes are optimized for sites that have a low, medium, or high annual average wind speed. However, the rotor size selected is always a compromise, and there are conditions in which the turbine does not perform optimally because the rotor is too big or too small.
Disclosed in U.S. Pat. No. 6,902,370 is a variable length wind turbine blade that allows for a large diameter in low winds and a small diameter in high winds. This is accomplished by having a root portion and a tip portion of the blades. The tip portion may be extended or retracted, depending on the amount of wind present.
To adjust the length of the blade and sense certain variables, there may be sensors and drives located on the tip portion of the blade, and these sensors must have power and control signals. A standard method of sending power and signals is to run wires. Running wires would be less than ideal because of the varied length of the distance between any point on the radially stationary part of a rotor and the radially varying blade tip portions. Wires long enough to reach when the tip portion is extended would slacken and get in the way when the tip portion is retracted. Having slack wires moving about in the blade causes unusual stress on the wires and the potential for the wires to be caught in other moving parts.
It would be advantageous to have a method of running power and control signals to a tip portion of a wind turbine blade in a manner that is adjustable depending on the degree of extension in the tip portion.